1. Technical Field
The present invention relates to a gas generating device, and particularly to an inflator for inflating an air bag to protect an occupant of a vehicle.
2. Description of the Prior Art
Inflators to expand an air bag, when a vehicle encounters a sudden deceleration, such as in a collision, are well known. The air bag restrains movement of an occupant of the vehicle during the collision. The air bag is inflated by gas generated by activation of gas generating material. The gas generating material is contained in the air bag inflator.
The inflator for an air bag which protects the driver of a vehicle is usually mounted in the vehicle steering wheel. Such an inflator is preferably small and light weight to minimize the rotating mass of the steering wheel assembly. The inflator should also, for ease of manufacture and low cost, have a minimal number of parts which are simple to assemble.
A commonly used gas generating composition is one containing an alkali metal azide and a metal oxide, such as cupric oxide or iron oxide. Ignition of the gas generating composition produces a hot gas stream. The hot gas stream will typically include nitrogen gas, molten metal, sodium oxide, a sodium salt of the metal, and metal nitride. The nitrogen gas carries the other reaction products, which are in the form of a residue or sinter.
A particular problem facing the air bag industry has been the development of an effective, low cost, mechanical filtering system for filtering residue and sinter from the hot gas stream. The filtering system must be capable of removing the residue or sinter from the gas before it enters the bag. The filtering system also should be capable of cooling the gas stream. A portion of the filtering system may have a reactant surface of an acidic oxide, such as silicon dioxide, to react with and neutralize the sodium oxide in the hot gas stream. To achieve these objectives, the filtering system must maintain a sealing zone where it contacts the inflator housing, so that the majority of the hot gas stream is directed through the filtering system.
U.S. Pat. No. 4,902,036 discloses a gas generator for a vehicle occupant restraint. The generator contains a plurality of filters including a final filter assembly which filters the nitrogen gas before the gas exits from the generator into an air bag. The final filter assembly comprises a first section of mesh screens. The mesh screens remove particulate reaction products from the gas stream. The final filter also has a second section of screens of smaller mesh size. The screen sections are separated by a spacer pad formed from a ceramic fiber that is a mixture of aluminum oxide and silicon dioxide. The spacer pad is sold under the trademark FIBERFAX by Sohio Carborundum, Inc. It is also known to have the foregoing final filter assembly seal against the generator housing by pressing graphite seals (not shown in the patent) between the housing and the upper and lower edges of the filter.
U.S. Pat. No. 4,878,690 also discloses a filter assembly used in a gas generator for a vehicle occupant restraint. The filter assembly comprises, in the direction of gas flow, a plurality of layers of 30 mesh stainless steel screen, a filter member for fine filtering the generated gas, and then another layer of 30 mesh stainless steel screen. The filter member can be an alumina-silica fiber paper such as LYTHERM ceramic fiber paper marketed by Mansville Co. Other examples of filter paper disclosed in the patent are silica fiber paper marketed by SANDTEX Corporation of Japan under the trademark "Sandtex" and a filter material sold by National Standard Co. of Corbin, Kentucky under the trademark "FIBREX". There is no disclosure in the patent as to how the filter assembly is sealed against the housing for the gas generator.